This invention relates to a method and apparatus for monitoring the surface level of a liquid using a contacting sensing means. More particularly, the invention relates to a method of using two movable electrodes for monitoring the surface level of a liquid, particularly useful with molten metal casting systems.
There are various methods known for sensing and measuring the liquid level on a continuous basis, generally in a contacting or noncontacting manner. Such systems have been proposed to measure the liquid level in a vessel, and particularly, of molten metal in a vessel, tundish, or casting mold.
Various noncontacting methods and devices may be based on eddy currents, laser beam reflections, and ultrasonics. When using eddy currents, variations induced within the coils of a sensing head either suspended above the liquid or incorporated in the main body of the vessel or casting mold itself is used. Another commonly proposed method employs laser beam reflections wherein a laser light is reflected off the surface of the liquid to a detector and measuring the relative angle between the laser and detector. When used with molten steel, however, the reflected light is disturbed due to the high temperature atmosphere, the turbulent state of the molten steel surface, and the flux powder normally applied to the exposed surface of the molten metal.
Another method which has been proposed is based on ultrasonics or sonic measurements produced by a transducer above the molten metal surface. Because of the high temperature atmospheres involved with the molten steel, both the laser method and ultrasonic method have large errors in measurement and cannot be used reliably.
Various contacting methods and apparatus have also been proposed. For example, it is known to use a spiral flow or a pressure in a bubble tube for measuring the level of the liquid metal. Still further, it has been proposed to use a single electrode for detecting the level of the molten metal; the position of the electrode depending upon the electrical resistance between the electrode and the molten steel.
As disclosed in Japanese Patent Publication No. 6059513, published Dec. 25, 1985, a method for detecting the position of the liquid surface is carried out by raising and lowering a single electrode at regular intervals in order to contact the liquid surface and thus measure the change in the position of the molten metal surface over time.
Although attempts have been made by another to use a fixed two-electrode probe system for monitoring the top of the pool of the liquid metal in the casting tundish, such a method was not able to dynamically track the position of the surface of the liquid metal and did not include a feedback controlled electrode system for monitoring the surface of the liquid metal.
While such methods and devices have been somewhat successful, they are either more expensive, less accurate, or not practical for use with molten metal or molten metal continuous casting operations.
What is needed is a method and apparatus for monitoring the surface level of a liquid, particularly molten metal, and more particularly molten metal having little or no slag layer. The method and apparatus should be suitable for use in the harsh environments of molten metal casting systems and be practical and yet inexpensive. The method and apparatus should be able to dynamically track the liquid surface and should be feedback responsive to monitor the liquid surface.